Large-area fiber composite with high fiber consistency

ABSTRACT

A method of manufacturing a fiber assembly, the method comprising: (a) providing a plurality of layers, each layer comprising sintered fibers of piezoelectric material aligned substantially parallel; (b) laminating the plurality of layers; and (c) applying a matrix material to the laminated layers to affix the layers and form a fiber assembly.

REFERENCE TO RELATED CASE

[0001] This case is based on provisional Application No. 60/196,427,filed Apr. 12, 2000, which is hereby incorporated by reference in itsentirety.

FIELD OF THE INVENTION

[0002] The present invention relates generally to assemblies of ceramicfibers, and, more specifically, to assemblies of piezoelectric materialsand the uses therefor.

BACKGROUND OF THE INVENTION

[0003] Piezoelectric materials are materials which have an electricalresponse to being deformed or stressed, and conversely, deform or“actuate” in response to an applied voltage. The ability ofpiezoelectric material to translate between electrical signals andphysical deformation renders them useful in various applications. Forexample, the inventors have identified applications in which thepiezoelectric materials are used for energy harvesting, actuating,sensing, transmitting/receiving waves, and combinations thereof. Theseapplications, however, often require the piezoelectric material to beformed into a fiber composite having a relatively large active surfacearea compared to traditional fiber composites. Traditional fibercomposites tend to be limited in size due to the low strength propertiesof commonly used piezoelectric materials, such as PZT (lead zirconiumtitanate).

[0004] Prior art approaches for overcoming the weakness of piezoelectricmaterials to produce a fiber composite having a relatively large surfacearea have been met with limited success. For example, a common approachfor providing fiber composites having a relatively large area involves adiced and filled piezoelectric substrate. More specifically, thepiezoelectric substrate is diced by forming a number of perpendicularchannels through it. Next, the channels are back filled with a plasticor other high-strength material. Although this approach strengthens thepiezoelectric substrate and allows for large active surfaces, it suffersfrom a number of significant shortcomings, one of the most significantbeing the occurrence of the Lamb wave mode of vibration. The Lamb wavemode of vibration is created by the lateral motion of the variouschannels which induces an overall resonance in the fiber composite.

[0005] The inventors have developed another approach to prepare fibercomposites which involves forming a “fiber rope” of piezoelectricmaterial in fiber form. More specifically, individual fibers ofpiezoelectric material are prepared using known processes such as thosedescribed in U.S. Pat. No. 5,827,797 (which has a common assignee to thepresent application), incorporated herein by reference. A bundle ofthese fibers then is braided or otherwise bound together and sintered.After sintering, an epoxy or other suitable matrix material can be addedto the sintered bundle to impart strength. At this point, individualfiber composites can be sectioned from the fiber rope. Each fibercomposite is a cross section of the fiber rope and has two planar,parallel opposing sides or active surfaces. The fibers are substantiallynormal to the active surfaces and thus their poling direction is normalto the active surface. Having the piezoelectric poling directionsubstantially normal to the active surface is a desirable property of afiber composite.

[0006] The use of fiber avoids the occurrence of the Lamb wave mode ofvibration since fibers tend to have narrow diameters, and, thus, theirradial secondary poling direction is minimal (see, e.g., U.S. Pat. No.5,869,189, incorporated herein by reference). In other words, the radialmovement of fibers tends to be very little—most of their polingdirection is along their length. Additionally, the random diameter andpacking of fibers also tends to minimize harmonics.

[0007] Although the use of fiber rope avoids the Lamb wave mode ofvibration, fiber ropes are nevertheless faced with other significantshortcomings. Perhaps the most significant shortcoming is theirlimitation in size. More specifically, it has been found that fiberropes greater than about 1¼ in diameter are difficult to produce due tothe lack of control over fiber consistency. More specifically, thefibers toward the perimeter of the rope tend to be more tightly packedthan the fibers toward the center. This condition worsens as thediameter of the rope increases.

[0008] Therefore, a need exists for a piezoelectric fiber compositewhich has a large active surface but which is not susceptible to Lambwave mode harmonics and is not limited in area. The present inventionfulfills this need among others.

SUMMARY OF THE INVENTION

[0009] The present invention provides for a piezoelectric fibercomposite which has a large active surface and a high degree of fiberconsistency. The fiber composite of the present invention avoids theaforementioned problems by using small subassemblies or thin layers offiber in which fiber consistency can be controlled to form a large fiberassembly having sufficient overall fiber consistency. In other words,the inventors have recognized that the most effective way of controllingthe fiber consistency of an entire assembly is by controlling the fiberconsistency of its smaller components. After the large fiber assembly isformed, thin fiber composites having opposing (active) surfaces can besectioned.

[0010] By sectioning the fiber composites from a large assembly ofconsistently arranged fibers, the present invention overcomes thevarious problems in the prior art. More specifically, since fibers arebeing used, issues with respect to Lamb wave mode harmonics are avoidedas discussed above with respect to the fiber rope. Furthermore, sincethe fiber assembly comprises the conglomeration of a number of smallercomponents in which fiber consistency can be readily maintained, thesize of the fiber assembly is not limited by fiber consistency. Theindividual fiber composites are sectioned off from the fiber assembly atthe desired thickness and thus have an active surface as large as anyside of the fiber assembly.

[0011] One aspect of the present invention is a method of preparing afiber assembly having a large cross section and fiber consistency. In apreferred embodiment, the method comprises: (a) providing a plurality oflayers, each layer comprising sintered fibers of piezoelectric materialaligned substantially parallel; (b) laminating the plurality of layers;and (c) applying a matrix material to the laminated layers to affix thelayers and form a fiber assembly. Having a large assemblage of fibersallows cross sections of the assemblage to be removed as fibercomposites. Accordingly, in a preferred embodiment, the method furthercomprises sectioning a portion from the fiber assembly wherein theportion has two opposing surfaces and contains fibers that aresubstantially normal to the opposing surfaces. The method alsopreferably comprises applying an electrode to each opposing surface.

[0012] Another aspect of the present invention is the product made fromthe process described above. In a preferred embodiment, the productcomprises a fiber composite comprising: (a) two opposing surfaces,wherein each opposing surface has an area greater than about 1.5 in²;(b) a plurality of piezoelectric fibers wherein the variation of fiberconcentration/cm³ of fiber composite throughout the fiber composite doesnot exceed 20% of the overall fiber concentration of the fibercomposite; and (c) a matrix material binding the fibers.

[0013] Another aspect of the present invention is the use of thepiezoelectric fiber composite described above. In a preferredembodiment, the fiber composite is used for energy harvesting, sensing,wave transmitting/receiving and combinations thereof.

BRIEF DESCRIPTION OF FIGURES

[0014]FIG. 1 shows a piezoelectric fiber composite of the presentinvention having a large surface area;

[0015]FIG. 2 shows a flow chart of a preferred method of preparing afiber composite of the present invention;

[0016]FIG. 3 schematically represents forming the fiber assembly of FIG.2 by laminating a number of planar layers; and

[0017]FIG. 4 shows a fiber assembly of the present invention fabricatedin accordance with the method depicted in FIG. 3 from which the fibercomposite of FIG. 1 is derived.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0018] Referring to FIGS. 1a & 1 b, preferred embodiments of the fibercomposite of the present invention are shown. Fiber composites 100 and100′ comprise a multitude of individual fibers 101 of piezoelectricmaterial held in a matrix material 104. Each fiber composite 100, 100′has opposing sides 102 and 103, also referred to herein as “activesurfaces,” which are substantially planar and parallel to one another.In the fiber composite of FIG. 1a, the fibers 101 are substantiallynormal to opposing sides 102 and 103, while in the fiber composite ofFIG. 1b, the fibers 101′ are substantially parallel to opposing sides102 and 103. Although the fibers are shown both normal and parallel tothe opposing sides, it should be understood that other configurationsare within the scope of the invention, for example, the fibers may be atangle (other than normal) to the opposing sides. Fiber composites 100,100′ also comprises electrodes 105 a and 105 b on each side 102 and 103from which extend leads 106 and 107, respectively. For illustrativepurposes, a section of electrode 105 a has been removed from side 102 ofeach fiber composite to reveal fibers 101, 101′.

[0019] An important feature of the fiber composite of the presentinvention is its controlled fiber characteristics, even over a largeactive surface. The term “fiber characteristics” refers to theconcentration, orientation, consistency, type, shape, size, andelectro-mechanical properties of the fibers in the fiber composite. Byprecisely controlling the fiber characteristics, the performance of thefiber composite can be precisely controlled.

[0020] The term “fiber concentration” herein refers to the fibervolume/composite volume and is typically stated as a percentage (e.g., afiber composite having a volume of 100 cm³ and a fiber volume 20 cm³ hasa fiber concentration of 20%). The fiber concentration in the fibercomposite 100 depends upon the application. For example, transmittingapplications tend to require higher fiber concentrations than receivingapplications. With respect to sonar applications, it has been found thatthe fiber concentration for the receiver is preferably about 10 to about20%, and more preferably about 18%, while the fiber concentration forthe transmitter is preferably about 25 to about 35% and more preferablyabout 28 to about 32%. It may be preferable to vary the fiberconcentration in a single composite between these two ranges to allowthe composite to both transmit and receive with optimum performance. Insuch an embodiment, the sections of the composite having differencefiber concentrations would be coupled to different sets of electrodesand, thus, operate somewhat independently of one another.

[0021] With respect to ultrasound applications, it has been found thatthe fiber concentration is preferably about 35 to about 45% and morepreferably about 38 to about 42%. With respect to sensors and actuators,it has been found that the fiber concentration is preferably at leastabout 50%, and, more preferably, at least about 75%. The fiberconcentration for other applications can be determined by one skilled inthe art in light of this disclosure without undue experimentation.

[0022] Although it may be preferable to vary the fiber concentrationacross the fiber composite between different sets of electrodes asmentioned above, it is generally preferred to maintain a high fiberconsistency for all the fibers associated with a given set ofelectrodes. The term “fiber consistency” as used herein broadly refersto the uniformity of fiber material/properties throughout the fibercomposite. The inventors submit that there are a variety of ways toquantify such fiber consistency and offer several approaches below asexamples, although the present invention is not necessarily limited byany one of these quantifications unless otherwise indicated.

[0023] A preferred measure of fiber consistency is variation in fiberconcentration per unit volume of fiber composite. Preferably, thevariation of the fiber concentration/cm³ of fiber composite does notexceed 20% of the composite's overall fiber concentration. Morepreferably, the variation of fiber concentration/cm³ of composite doesnot exceed 10% of the overall fiber concentration. Even more preferably,the variation of the fiber concentration/cm³ of composite does notexceed 5% of the overall fiber concentration. For example, in the morepreferred embodiment, in which the variation of the fiberconcentration/cm³ of composite does not exceed 10% of the overall fiberconcentration, if the fiber concentration of the composite is 30%, thanthe variation in fiber concentration is preferably no greater than about3%/cm³ of fiber composite, or, in other words, the fiber concentrationis no less than 27%/cm³ and no greater than 33%/cm³.

[0024] Another measure of fiber consistency is the variation in thedistance between adjacent fibers. As used herein, the distance betweenfibers is centerline distance. Preferably, the variation in distancedoes not exceed 30% of the average difference between fibers for theentire composite, more preferably, the variation in distance does notexceed 20% of the average difference between fibers, and, even morepreferably, the variation in distance does not exceed 10% of the averagedifference between fibers. For example, in the more preferred embodimentin which the variation in distance does not exceed 20% of the averagedifference between fibers, if the average overall distance betweenfibers is 300 m, than the variation between fibers cannot exceed 60 km.

[0025] Rather than basing fiber consistency on a physical parameter ofthe fiber composite, fiber consistency may be related to the variationin properties across the fiber composite. Of primary importance in afiber composite of piezoelectric material is its electro-mechanicalresponse. The term “electro-mechanical response” as used herein refersto the mechanical response (i.e., actuation) of the fiber for a givenelectrical field, or, conversely, an electrical response for a givenfrequency of actuation. The actual measurement may be, for example, themagnitude of the response (e.g., volts, mm), the gain of the response(i.e., input:output), or the frequency of the response (e.g., Hz). Thus,in a preferred embodiment, the variation in the electro-mechanicalresponse per cm³ of fiber composite does not exceed about 25% of theoverall electro-mechanical response of the fiber composite. Morepreferably, the variation of the electro-mechanical response/cm³ ofcomposite does not exceed 15% of the overall electro-mechanical responseof the fiber composite. Even more preferably, the variation of theelectro-mechanical response/cm³ of composite does not exceed 10% of theoverall electro-mechanical response of the fiber composite.

[0026] Another important feature of the fiber composite of the presentinvention is its large active surface (e.g., sides 102 and 103). Thissurface is larger than that achievable using fiber rope of equal fiberconsistency. For example, as mentioned above, a fiber rope of adequatefiber consistency tends to be limited in diameter to about 1¼ in. whichcorresponds to a cross-sectional area of about 1.2 in². Accordingly, theactive surface of the fiber composite of the invention has an areapreferably greater than about 1.2 in², more preferably, greater thanabout 1.5 in², even more preferably, greater than about 3 in², and,still more preferably, greater than about 5 in².

[0027] Referring to FIG. 2, a flow chart is shown depicting themanufacturing steps to form a preferred fiber composite of theinvention. In step 201, the fiber is prepared. Techniques for preparingpiezoelectric fiber are known in the art, although preparing the fibersaccording to the spinning techniques described in U.S. Pat. No.5,827,797 is preferred.

[0028] The fibers may be made of various electro-ceramic materialsincluding, for example, all members of the PZT (lead zirconium titanate)family, lead niobate (PbNbO₆), lead titanate (PbTiO₃), barium titanate(BaTiO₃), electrostrictive materials, e.g. magnesium niobate (MgNbO₆),sodium bismoth titanate (pure or co-doped), other lead-based ceramicsdoped with lanthanum, tin, or niobium, and shape-memory piezoelectricmaterials (e.g., Pb₃ MgNb₂O₆), and relaxor materials (ferroelectric/nonferroelectric). Preferably, the piezoelectric material is PZT. Suitablefibers made of piezoelectric material, such as PZT, are commerciallyavailable from Advanced Cerametrics, Lambertville, N.J. It should beunderstood, that although piezoelectric fibers are discussed herein indetail, the present invention may be practiced to manufacture anassembly of any type of ceramic fiber, including those disclosed in U.S.Pat. No. 5,827,797.

[0029] The diameter of the fibers used can vary depending upon theapplication. For example, lower frequency applications generallycorrespond to larger diameters. Additionally, it may be beneficial touse fibers of different diameters to effect a multi-frequency fibercomposite. Given these considerations, fiber diameters ranging fromabout 5 to about 300 m are typical. Although circular fibers arepreferred, it should be understood that fiber geometry may be variedwithin the scope of the invention. For example, the fibers may be formedwith a square or rectangular cross section by extruding fibers through asquare die. Such a geometry may be preferred in fact to effect highfiber concentrations since square fibers tend to pack tighter thancircular fibers.

[0030] The length of the fiber may be adjusted depending on the size ofthe desired fiber assembly. In a preferred embodiment, the fibers arecut from a spool of fiber obtained using the wet spinning techniquedisclosed in U.S. Pat. No. 5,827,797 mentioned above. More specifically,the spool is divided into arcuate sections, each section containing manyfibers, the exact number corresponding to the number of turns on thespool.

[0031] One the fiber are manufactured, then a laminate is formed in step202. To this end, the fibers prepared in step 201 are laid in a tray insubstantially parallel alignment. To effect the substantially parallelalignment, it may be preferable to comb the fibers. It should beunderstood, however, that an exact parallel alignment is not necessary,as the applicants have found that this configuration increases thelikelihood of producing harmonics in the fiber composite andexperiencing a Lamb wave mode of vibration.

[0032] The thickness of the layer depends on a number of factors one ofwhich being the desired degree of control over fiber consistency.Generally, thick layers correspond to less control over fiberconsistency. One skilled in the art can readily determine the thicknesscorresponding to the desired fiber consistency. Another factor relatingto layer thickness is fiber diameter. Generally, larger diameters fiberscorrespond to thicker layers. Given these considerations, a typicallayer comprises a stack of fibers ranging from about 4 to about 30fibers thick which generally corresponds to a thickness of about 40 toabout 500 m.

[0033] Once the fibers have been formed into a layer and adequatelyarranged, the fibers are sintered in step 203 using conventionaltechniques and equipment. For example, in preparing a layer of PZTfibers, it has been found that adequate results can be achieved attemperatures ranging from about 1150 to about 1300° C. in a lead-richatmosphere. The sintering time can range significantly, for example,from about 45 minutes to 10 hrs, depending upon the thickness of thelayer and desired degree of sintering. In addition to homogenizing theparticles of PZT material into a fiber, there is also a certain amountof cross linking occurring between the various fibers such that, aftersintering, the fibers in the layer are coupled. This facilitateshandling as the layer is now one piece.

[0034] Following sintering in step 203, the laminates are assembled andformed into a fiber assembly in step 204. A schematic of this assemblystep is shown in FIG. 3. As shown, the sintered layers 301 are alignedwith their planar surfaces 302, 303 adjacent one another and are thenpressed together. In this embodiment, the fibers are orientated alongthe x-axis, although they may just as well be aligned along the y axisinstead. To hold the layers together in this configuration, a matrixmaterial 304 is used.

[0035] The preferred matrix material is a soft and deformable polymer.Preferably, the Young's modulus is below about 50 GPa, e.g., about 3-10GPa. The Young's modulus of the polymer is preferably at least 5 or 10times less than the Young's modulus of the piezoelectric fibers, whichfor most materials is about 60 GPa. Polymers which may be used includethermosetting and thermoplastic families of polymers, including epoxies,polyamides and cross-linked polyamides, polyvinyl chlorides,polyethylenes, and active polymers that exhibit electro-mechanicalcoupling, e.g. polyvinyl difluoride (PVDF). The polymer may also includeadditives to achieve desired elastic and dielectric properties. Forexample, to increase the dielectric constant of the polymer,high-dielectric-constant particles or fibers composed of graphite,metal, ceramic, or electroceramic materials may be added. Furthermore,in ultrasound or sonar applications, it is preferable to introduceadditives or use a polymer which is acoustically matched to thefrequency of the application. Preferred matrix materials include epoxypolymers.

[0036] When applying the matrix material to the assembly of sinteredlayers, preferably a vacuum is drawn such that air is evacuated from theinterstices of the sintered layers and the matrix material is drawn in.To this end, the assembly is contained in a flexible vacuum bagconnected to a vacuum pump. When a vacuum is drawn in the bag, the bagcollapses around the mold which facilitates compression during curing ata pressure of about one atmosphere.

[0037] In addition, the vacuum condition within the bag allows airbubbles to be removed from the epoxy prior to curing to insure maximumcompaction. Typically, a vacuum of approximately 30 in Hg is applied tothe system overnight without heat to minimize voids. It is desirable tokeep void content in the composite to a minimum since the dielectricstrength of epoxy and piezoelectric fibers is four to five times higherthan that of air, and voids present locations across which the appliedvoltage may arc. Consequently, high void concentrations in the compositemay make it difficult to reach the poling voltages

[0038] In an alternative embodiment, layers 301 are impregnated withmatrix material individually and then laminated to form the compositeassembly by adhering the layers together and/or by pressing the layerstogether and heating them such that the matrix material in theindividual layers reflows and combines with the matrix material ofadjacent layers. It should be understood that other laminationtechniques or a combination of one or more of techniques is within thescope of invention.

[0039] Curing is well known in the art and can be accomplished byheating the structure to a temperature and applying pressure for a timeperiod appropriate for a particular polymer system.

[0040] Referring now to FIG. 4, the composite structure formed from theprocess described above is shown. It is worthwhile to note that althoughindividual layers 301 are depicted in assembly 400, this is done forillustrative purposes and the interface between the various layers maylikely be indistinguishable once assembled. The fiber consistencydescribed above with respect to fiber composite 100 is manifested in thefiber assembly 400.

[0041] Once the fiber assembly 400 is formed, individual fibercomposites may be sectioned from it in step 205. For example, assumingthat the fibers are oriented along the x axis as in FIG. 3, the dottedlines 402 and 403 on fiber assembly 400 indicate the cross sectionscorresponding to the surfaces 102 and 103 of fiber composite 100 of FIG.1a in which the fibers are substantially normal to the opposingsurfaces. The dotted lines 402′ and 403′ on fiber assembly 400 indicatethe cross sections corresponding to the surfaces 102 and 103 of fibercomposite 100′ of FIG. 1b in which the fibers are substantially parallelto the opposing surfaces.

[0042] It should be understood that the fiber composites 100, 100′ ofany thickness can be sectioned from assembly 400. Thus, a multitude offiber composites 100 can be derived from a single fiber assembly 400.Conversely, it is well within the scope of the invention for a fibercomposite to comprise the entire fiber assembly 400.

[0043] The thickness of the fiber composite depends on the orientationof the fibers in the composite and on the application. To achieveoptimum performance, the thickness of the fiber composite 100′ tends tobe less than that of fiber composite 100. More specifically, to achievethe highest degree of response from a fiber, it is preferred that theelectrodes are intimately close to and more preferably contacting thefibers. Since the fibers are parallel to the electrodes in the fibercomposite 100′, only the fibers along the opposing surfaces are intimatewith the electrodes. If the composite is more than two fibers thick, theinterior fibers will be separated from the electrodes and, thus, theirelectrical coupling to the electrodes will be severely diminished.Therefore, for composite 100′, it is generally preferred that it not besubstantially more than two fibers thick.

[0044] The application of the fiber composite also plays a significantrole in determining the thickness of the fiber composite. For example,space and flexibility requirements may necessitate a fairly narrow fibercomposite 100. On the other hand, to maximize the lengthwise poledirection of the fibers, it may be desirable to increase the thicknessof the fiber composite 100. It is well known that increasing the lengthof the fibers along the pole direction will enhance their energyharvesting and actuating properties. Given these considerations, typicalfiber composite thickness range from about 4 m to about 1 in.

[0045] Once the fiber composite has been sectioned from the compositeassembly, it may be fitted with electrodes in step 206. The electrodemay be any conductive surface and suitable configurations thereof arewell known in the art, including, for example, interdigitizedconfigurations. The electrode is preferably a thin, flexible conductivelayer which does not restrain the composite or the structural componentduring actuation. Suitable conductive materials include, for example,silver, aluminum, copper, and gold, as well as non-metallic conductorssuch as conductive polymers. Of these materials, silver is preferred.The conductive material may be applied to the fiber composite in variousways including for example, vapor deposition, sputtering, ink film, andelectron beam evaporation at low power, or the electrode layers may beformed of a thin polymer substrate coated with an ultra-thin layer ofmetal. Preferably, the electrode layer is applied by sputtering.

[0046] The piezoelectric fiber composites may be poled in step 207 usingknown techniques. For example, they may be poled by placing them into ahot oil bath, typically at a temperature of 80° C. The hot oil servestwo functions: (1) arcing and dielectric breakdown are reduced by thepresence of the oil, and (2) the heat facilitates alignment of thedipoles. Poling is typically carried out for a certain time and voltagelevel typically depending on the size of the fiber composite. Followingpoling, the fiber composite can be activated by attaching the electrodesto a voltage source using wires or conductive tape.

[0047] Once prepared, the piezoelectric fiber composite may be coupledto control circuitry in step 208. Preferably, the fiber composite iscoupled to a control unit by making an electrical connection to theconductive electrode layers. The control unit preferably includes ananalog-to-digital converter to process electrical signals from the fibercomposite layer, a computer to analyze the processed signal from thesensor, and an amplifier which receives a signal from the computer andsends an actuating electrical signal to the fiber composite. Themagnitude, frequency, waveform, etc. of the actuating signal isdetermined based on the nature of the desired displacement for theparticular application.

[0048] Once the fiber assembly is interfaced with its controlelectronics, the piezoelectric material of fibers 101 will generate anelectrical potential between electrodes 105 a and 105 b upon beingphysically deformed. This electrical potential is conducted throughleads 106 and 107 and may provide electrical energy for energyharvesting purposes or a signal for measurement purposes. Conversely,when an electrical potential is applied between electrodes 105 a and 105b across the fibers 101′, the fibers deform or “actuate.”

[0049] The inventors have recognized a wide range of applications usingthe fiber composite of the present invention given its relatively largearea and high fiber consistency. For illustrative purposes, theseapplications have been categorized below in terms of energy harvesting,wave transmitting/receiving, sensing and combinations of energyharvesting/sensing and actuation. It should be understood that thiscategorization is for illustrative purposes only and should not beconstrued to limit the scope of the invention.

[0050] 1. Energy Harvesting

[0051] The large active surface area obtainable with the fiber compositeof the present invention makes it particularly well suited forharvesting energy. More specifically, since the fiber composite may beconfigured to have a large active surface, it casts a “wide net” tocollect energy. Thus, in applications in which the energy is dispersedover an area, the composite of the invention is able to convert theenergy into electrical power. For example, a thin fiber composite may beused in the sole of a shoe such that every time a user takes a step andapplies the weight of his body against the fiber composite, voltage willbe generated. The electrical response may be stored in a capacitor orother type of energy storage means for use in powering accessories suchas lights, radios, boot heaters, and battery chargers.

[0052] 2. Wave Transmitting/Receiving

[0053] Given its large active surface area, flexible nature, andcontrolled fiber characteristics, the fiber composite of the presentinvention is particularly well suited for wave transmitting/receiving.For example, a fiber composite may be molded into the nose cone of asubmarine. In a preferred embodiment, the fiber composite has fibers ofdifferent diameters to operate at different frequencies. Accordingly, asingle fiber composite may contain both the transmitter and receiver,thereby minimizing components and reducing space requirements which isparamount on a submarine.

[0054] Another example of the composite's versatility is in the field ofultrasound bone-healing applications. Preferably, a thin, flexible fibercomposite is used to conform to a person's body and thereby providedirect contact between the fiber composite and the skin, therebyminimizing air gaps which attenuate ultrasound signals.

[0055] Yet another example of the composite's use is underwaterlistening for oil exploration and fish finding as well as underwaterwave transmitting for attracting fish.

[0056] 3. Sensing

[0057] The versatility and robust nature of the composite of the presentinvention also makes it ideal for sensing applications in which thesensor needs to conform to an infrastructure or housing. For example, athin fiber composite may be stitched into clothing or integrally formedtherein in contact sports for scoring purposes or for determining theseverity of impacts. For example, such fiber composites may beincorporated into fencing attire to provide reliable indications ofsword contact and/or the severity of the contact.

[0058] Another example of a sensor application involves theincorporation of the fiber composites into highway structures such asbridges and roads to monitor stress thereon and to measure vehicletraffic and weight.

[0059] 4. Combination Sensing/Energy Harvesting and Actuating

[0060] The inventors have recognized that integrating the sensing,actuation and energy harvesting capabilities of piezoelectric materialsallows the composites to be self sufficient in certain applicationwithout the need for external power or actuation control. For example,such fiber composites may be used in air bag deployment configurations.In a preferred embodiment, the fiber composite acts as an accelerometerto sense rapid deceleration, it acts as an energy harvesting deviceobtain energy from the deceleration to provide power to controlcircuitry, and it acts as an actuator by deploying the air bag followinga signal from the control circuitry.

[0061] Another example of an integrated system involves using fibercomposites to harvest vibration energy. This vibration energy is thenused to power circuitry which controls the actuation of the composite todampen vibration. Particular examples include using fiber composites todamped vibration in sports equipment, such as tennis rackets, or usingfiber composites in a self-propelled device, such as a rocket, toharvest vibration energy or the energy of a tail flapping in the wind topower control circuitry of the device.

What is claimed is:
 1. A method of manufacturing a fiber assembly, said method comprising: providing a plurality of layers, each layer comprising sintered fibers of piezoelectric material aligned substantially parallel; laminating said plurality of layers; and applying a matrix material to the laminated layers to affix said layers and form a fiber assembly.
 2. The method of claim 1, further comprising: sectioning a portion from said fiber assembly.
 3. The method of claim 2, wherein said portion has two opposing surfaces and contains fibers that are substantially normal to said opposing surfaces.
 4. The method of claim 2, wherein said portion has two opposing surfaces and contains fiber that are substantially parallel to said opposing surfaces.
 5. The method of claim 2, further comprising: applying at least one electrode to each opposing surface.
 6. The method of claim 5, wherein a plurality of interigitized electrodes are applied.
 7. The method of claim 1, wherein laminating said planar layers comprises interleaving planar layers of varying fiber characteristics.
 8. The method of claim 7, wherein said layers of varying fiber characteristics have different fiber concentrations.
 9. The method of claim 7, wherein said layers of varying fiber characteristics have fibers of different average diameters.
 10. The method of claim 7, wherein a different set of electrodes is applied to said layers of varying fiber characteristics.
 11. The method of claim 1, wherein said layers have substantially similar fiber characteristics.
 12. The method of claim 1, further comprising poling said sectioned portion.
 13. The method of claim 1, wherein said piezoelectric material is at least one of PZT (lead zirconium titanate), lead niobate (PbNbO₆), lead titanate (PbTiO₃), barium titanate (BaTiO₃), sodium bismoth titanate (pure or co-doped), lead-based ceramics doped with lanthanum, tin, or niobium, electrostrictive materials, memory piezoelectric materials, or relaxor materials.
 14. The method of claim 1, wherein each opposing side of said portion has an area greater than about 1.5 cm².
 15. The method of claim 1, wherein the variation of fiber concentration in no greater than about 20%/cm³.
 16. A fiber assembly made from the method of claim
 1. 17. A portion made from the method of claim
 2. 18. A fiber composite comprising: two opposing surfaces wherein each opposing surface has an area greater than about 1.5 in²; a plurality of piezoelectric fibers wherein the fiber concentration/cm³ varies no greater than about 20% of the overall fiber concentration of fiber composite; and a matrix material binding said fibers.
 19. The fiber composite of claim 18, wherein said fibers are normal to said opposing surfaces.
 20. The fiber composite of claim 18, wherein said area is no less than about 2.5 in^(2.) 